the eix database is built from the ebuilds available in the portage tree (and overlays). if an ebuild gets entirely removed from the portage tree, eix will no longer list it, and therefore not able to help you find it.

In you case, xulrunner was used by firefox, but more recent versions do not need it anymore (as is provided directly by firefox), but at the same time it was not mandatory to remove it. so over time, when old version which required xulrunner got removed from the portage tree, xulrunner was also removed, and thus gets no longer listed in eix.

I hope that clarifies it.

V._________________read the portage output!
If my answer is too concise, ask for an explanation.